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Re: [TenTec] OT: Indoor Antenna

To: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] OT: Indoor Antenna
From: Steve Hunt <steve@karinya.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 03 Dec 2010 15:25:46 +0000
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>
To illustrate Ken's point, here are some measurements I made on the 
feedpoint resistance of my 160m 40ft top-loaded vertical as I added 
radials. Rrad for this antenna should be about 5 Ohms.

6ft Ground rod only: 33 Ohms, Efficiency = 15%, VSWR = 1.5
Ground rod + 5 radials: 13 Ohms, Efficiency = 38.5%, VSWR = 3.8
Ground rod + 10 radials: 9 Ohms, Efficiency = 55.5%, VSWR = 5.5
Ground rod + 15 radials: 7 Ohms, Efficiency = 71.5%, VSWR = 7.1

I ended up with a 9:1 impedance transformer at the feedpoint to give me 
a reasonable match to 50 Ohms. With just the ground rod I would have had 
a reasonable match without any transformer, but the efficiency would 
have been 15% (-8.2dB).

Estimated ground loss resistance for the 6ft Ground rod on its own was 
28 Ohms.

Steve G3TXQ

On 03/12/2010 07:01, Ken Brown wrote:
> A quarter wavelength monopole with a really good counterpoise/ ground
> radial system has a feed point radiation resistance of around 37 ohms
> and very little loss resistance. That will give you an SWR of about
> 1.4:1. If you use just a ground rod and have about 13 ohms of resistive
> loss in the ground system, the series equivalent feed point impedance
> will be about 50 ohms, and the SWR will be very near 1:1. Some people
> think this is better. It does "tend to make the matching easier."
> I don't know what kind of ground you have, or whether your ground system
> resistance would be 13 ohms with just a ground rod. The point I am
> trying to make is that a really efficient vertical is likely to have a
> higher SWR than a partially radiating dummy load. Many hams do choose a
> lo
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